Microlithographic fabrication of structures

ABSTRACT

Asymmetric structures formed on a substrate and microlithographic methods for forming such structures. Each of the structures has a first side surface and a second side surface, opposite the first side surface. A profile of the first side surface is asymmetric with respect to a profile of the second side surface. The structures on the substrate are useful as a diffraction pattern for an optical device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.15/702,175, filed on Sep. 21, 2017, which claims the benefit of thefiling date of U.S. Provisional Application No. 62/397,604, filed onSep. 21, 2016. The contents of U.S. Application Nos. 62/397,604 and Ser.No. 15/702,175 are incorporated herein by reference in their entirety.

TECHNICAL FIELD OF THE INVENTION

This invention relates to micro- and nano-structures having desiredgeometries, and to microlithographic methods of fabricating suchstructures.

BACKGROUND OF THE INVENTION

Nano-fabrication includes the fabrication of very small structures thathave features on the order of 100 nanometers or smaller. One applicationin which nano-fabrication has had a sizeable impact is in the processingof integrated circuits. The semiconductor processing industry continuesto strive for larger production yields while increasing the circuits perunit area formed on a substrate, therefore nano-fabrication becomesincreasingly important. Nano-fabrication provides greater processcontrol while allowing continued reduction of the minimum featuredimensions of the structures formed. Other areas of development in whichnano-fabrication has been employed include biotechnology, opticaltechnology, mechanical systems, and the like.

Nano-fabrication can include processing of substrates to form variousshaped micro- and nano-structures on or in the substrate. One exampleprocess for forming such structures is imprint lithography.

SUMMARY OF THE INVENTION

This specification relates to forming micro- and nano-structures thathave asymmetric profiles. Asymmetric structures may, for example, beuseful in fabricating more efficient diffraction patterns for opticalwaveguides. Implementations of the present disclosure include a methodfor fabricating asymmetric structures by selectively applying a maskingmaterial to a substrate and etching portions of the substrate that arenot covered by the masking material. For example, the masking materialcan be applied to regularly shaped discrete structures in a manner suchthat the subsequent etching sculpts the structures into an asymmetricshape.

In general, innovative aspects of the subject matter described in thisspecification can be embodied in methods that include the actions offorming a plurality of discrete structures extending from a commonsurface on a substrate. Applying a masking material over the structuresunder conditions that cause the masking material to asymmetrically coverthe structures such that at least a portion of one side of eachstructure is free of the masking material. Etching an area of thestructures that is not covered by the masking material. And, strippingthe masking material from the structures. This and other implementationscan each optionally include one or more of the following features.

In some implementations, the conditions that cause the masking materialto asymmetrically cover the structures comprise inclining the substrateat a non-normal angle to a deposition direction from which the maskingmaterial is applied by a deposition system.

In some implementations, the plurality of structures include a firstregion of structures and second region of structures. Applying themasking material to the structures asymmetrically can include maskingthe second region of structures while applying the masking material tothe first region of structures. In some implementations, applying themasking material to the structures asymmetrically can include maskingthe first region of structures while applying the masking material tothe second region of structures.

In some implementations, etching an area of the structures comprisesperforming one of a wet etch process, a dry etch process, or an ion beametching process.

In some implementations, the portion of one side of each structure thatis free of the masking material is a first portion, and the methodincludes applying a metal catalyst layer for metal assisted chemicaletching (MACE) over the structures under second conditions that causethe metal catalyst layer to asymmetrically cover the structures suchthat at least a second portion of one side of each structure is free ofthe metal catalyst layer. In some implementations, the second portion isdifferent from the first portion. In some implementations, theconditions that cause the masking material to asymmetrically cover thestructures comprise inclining the substrate at a first non-normal angleto an direction from which the masking material is applied by adeposition system, and the second conditions that cause the metalcatalyst layer to asymmetrically cover the structures comprise incliningthe substrate at a second non-normal angle to an direction from whichthe metal catalyst layer is applied by the deposition system. The secondnon-normal angle can be different from the first non-normal angle.

In some implementations, the structures are etched after stripping themasking material from the substrate.

In some implementations, the structures have a square profile,rectangular profile, trapezoidal profile, or a triangular profile beforethe step of etching the area of the structures.

In some implementations, the masking material includes one of: Cr, Ti,SiO₂, Al₂O₃, ZrO₂, Ag, Pt, or Au. In some implementations, thestructures include one of: Si, SiO₂, a polymer material, or anorganic-inorganic hybrid material. In some implementations, thestructures are nano-structures. In some implementations, the structuresare micro-structures.

Another general aspect can be embodied in a substrate that includes aplurality of structures on the substrate, each of the structures havinga first side surface and a second side surface, opposite the first sidesurface, and where a profile of the first side surface is asymmetricwith respect to a profile of the second side surface. This and otherimplementations can each optionally include one or more of the followingfeatures.

In some implementations, the structures include one of: Si, SiO₂, apolymer material, or an organic-inorganic hybrid material. In someimplementations, the structures are nano-structures. In someimplementations, the structures are micro-structures.

In some implementations, the plurality of structures include a pluralityof first structures having a first asymmetric profile and a plurality ofsecond structures having a second asymmetric profile that is differentfrom the first asymmetric profile.

In some implementations, the substrate is included in an optical device.In some implementations, the optical device is an optical wave guide. Insome implementations, the optical device is a pair of augmented realityglasses. In some implementations, the plurality of structures form adiffraction pattern.

Particular implementations of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages. Implementations of the present disclosure mayimprove the fabrication of micro- and nano-patterns of varying profilestructures for large patterns of structures. Implementations may permitthe fabrication of structures that have varying profiles over differentregions of a substrate (e.g., an Si wafer). Implementations may permitthe fabrication of more efficient wave (e.g., optical) diffractionpatterns. Implementations may permit the manufacture of mechanicallystable micro/nano-structures with a high aspect ratio.

As used herein, the terms “micro,” “micro-structure,” and“micro-feature” represent structures or features of a structure thathave at least one dimension that is less than or equal to 50micrometers.

As used herein, the terms “nano,” “nano-structure,” and “nano-feature”represent structures or features of a structure that have at least onedimension that is less than or equal to 500 nanometers.

The details of one or more embodiments of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other potential features, aspects, and advantages ofthe subject matter will become apparent from the description, thedrawings, and the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a simplified side view of a lithographic system.

FIG. 2 illustrates a simplified side view of a substrate having apatterned layer positioned thereon.

FIGS. 3A-3C illustrate simplified side views of substrates havingexample asymmetric structures patterned thereon.

FIG. 4 illustrates an example process for fabricating asymmetricstructures in accordance with implementations of the present disclosure.

FIG. 5 illustrates a second example process for fabricating asymmetricstructures in accordance with implementations of the present disclosure.

FIG. 6 illustrates a third example process for fabricating asymmetricstructures in accordance with implementations of the present disclosure.

FIGS. 7A-7C illustrates an example process for asymmetrically applying amasking material to structures on a substrate in accordance withimplementations of the present disclosure.

FIG. 8 shows a flowchart of an example method for fabricating asymmetricstructures in accordance with implementations of the present disclosure.

FIGS. 9A-9B show example devices in which asymmetric structures can beused.

DETAILED DESCRIPTION

Various examples of fabricating micro- and nano-structures that haveasymmetric profiles are described below. Asymmetric structures may, forexample, be useful in fabricating more efficient diffraction gratingsfor optical waveguides. Generally, these examples include selectivelyapplying a masking material to a substrate and etching portions of thesubstrate that are not covered by the masking material. For example, themasking material can be applied to regularly-shaped discrete structuresin a manner such that the subsequent etching sculpts the structures intoan asymmetric shape.

FIG. 1 illustrates an imprint lithography system 100 that forms a reliefpattern on a substrate 102. The substrate 102 may be coupled to asubstrate chuck 104. In some examples, the substrate chuck 104 caninclude a vacuum chuck, a pin-type chuck, a groove-type chuck, anelectromagnetic chuck, and/or the like. In some examples, the substrate102 and the substrate chuck 104 may be further positioned on an airbearing 106. The air bearing 106 provides motion about the x-, y-,and/or z-axes. In some examples, the substrate 102 and the substratechuck 104 are positioned on a stage. The air bearing 106, the substrate102, and the substrate chuck 104 may also be positioned on a base 108.In some examples, a robotic system 110 positions the substrate 102 onthe substrate chuck 104.

The substrate 102 can include a planar surface 111 positioned oppositethe substrate chuck 104. In some examples, the substrate 102 can beassociated with a thickness that is substantially uniform (constant)across the substrate 102.

The imprint lithography system 100 further includes an imprintlithography flexible template 112 that is coupled to one or more rollers114, depending on design considerations. The rollers 114 providemovement of a least a portion of the flexible template 112. Suchmovement may selectively provide different portions of the flexibletemplate 112 in superimposition with the substrate 102. In someexamples, the flexible template 112 includes a patterning surface thatincludes a plurality of structures, e.g., spaced-apart recesses andprotrusions. However, in some examples, other configurations ofstructures are possible. The patterning surface may define any originalpattern that forms the basis of a pattern to be formed on substrate 102.In some examples, the flexible template 112 may be coupled to a templatechuck, e.g., a vacuum chuck, a pin-type chuck, a groove-type chuck, anelectromagnetic chuck, and/or the like.

The imprint lithography system 100 may further comprise a fluid dispensesystem 120. The fluid dispense system 120 may be used to deposit apolymerizable material on the substrate 102. The polymerizable materialmay be positioned upon the substrate 102 using techniques such as dropdispense, spin-coating, dip coating, chemical vapor deposition (CVD),physical vapor deposition (PVD), thin film deposition, thick filmdeposition, and/or the like. In some examples, the polymerizablematerial is positioned upon the substrate 102 as a plurality ofdroplets.

Referring to FIGS. 1 and 2, the imprint lithography system 100 mayfurther comprise an energy source 122 coupled to direct energy towardsthe substrate 102. In some examples, the rollers 114 and the air bearing106 are configured to position a desired portion of the flexibletemplate 112 and the substrate 102 in a desired positioning. The imprintlithography system 100 may be regulated by a processor in communicationwith the air bearing 106, the rollers 114, the fluid dispense system120, and/or the energy source 122, and may operate on a computerreadable program stored in a memory.

In some examples, the rollers 114, the air bearing 106, or both, vary adistance between the flexible template 112 and the substrate 102 todefine a desired volume therebetween that is filled by the polymerizablematerial. For example, the flexible template 112 contacts thepolymerizable material. After the desired volume is filled by thepolymerizable material, the energy source 122 produces energy, e.g.,broadband ultraviolet radiation, causing the polymerizable material tosolidify and/or cross-link conforming to shape of a surface of thesubstrate 102 and a portion of the patterning surface of the flexibletemplate 112, defining a patterned layer 150 on the substrate 102. Insome examples, the patterned layer 150 may comprise a residual layer 152and a plurality of structures shown as protrusions 154 and recessions156.

FIGS. 3A-3C illustrate simplified side views of substrates 102 havingpatterned layer 150 with example asymmetric structures 300, 320, 340patterned thereon. More specifically, FIGS. 3A and 3B illustrate examplestructures 300, 320 that have a “chair-like” asymmetric profile and FIG.3C illustrates example structures 340 that have an asymmetric triangularprofile. The structures 300, 320, 340 each include a first side surface302 and a second side surface 304. As shown, the second side surface 304of each structure 300, 320, 340 is fabricated to be asymmetric to therespective first side surface 302.

The patterned layer 150 and structures 300, 320, 340 can be fabricatedusing materials including, but not limited to, Si, SiO₂, polymermaterials, or organic-inorganic hybrid materials. An example,organic-inorganic hybrid material is a film or patterned layer composedof 2,4,6,8-tetramethylcyclotetrasiloxane exposed to Ar and oxygen basedplasma under atmospheric or low pressure conditions which forms a carbonpolymer layer with methyl groups and silicon oxide. Another example,organic-inorganic hybrid material is hexachlorodisilane which can beused to deposit silicon nitride on a substrate. In another example,hexachlorodisilane can be used with Ar/O₂ plasma to deposit siliconoxy-nitride to form a clear film of high index (n<1.7). These layers canbe deposited and etched or deposited directly over relief structures. Inaddition, the structures 300, 320, 340 can be, for example, micro- ornano-structures.

In some implementations, a plurality of different types of asymmetricstructures 300, 320, 340 are included on a single substrate. Forexample, substrate 102 can include a patterned layer 150 that isfabricated to include a structures 300 in a first region, structures 320in a second region, and structures 340 in a third region.

In some implementations, the asymmetrically profiled structures such asstructures 300, 320, 340 are more mechanically stable while stillproviding a high aspect ratio geometry than some symmetric structuressuch as tall and narrow rectangular structures. For example, a highaspect ratio profile is desirable for fabricating a glancing angledeposition (GLAD) based imprints for wire grid polarizers over rigid orplastic substrates. However, symmetric structures that have a highaspect ratio (e.g., as shown in FIG. 2) may be less mechanically stable,whereas, mechanical stability can improved in structures that use anasymmetric profile. Furthermore, a greater volume of metal 310 (e.g.,Al) can be asymmetrically deposited on a stable asymmetric profile(e.g., as shown in FIG. 3A) than on a less stable symmetric structure.In some examples, the metal 310 can also be packed higher verticallywhen deposited on an asymmetric structure than on symmetric structures.

FIG. 4 illustrates an example process 400 for fabricating asymmetricstructures in accordance with implementations of the present disclosure.At step (402), a plurality of discrete structures 420 are formed on asubstrate 102. The structures 420 can be formed by, for example, imprintlithography, photo lithography and etching, or other appropriatefabrication techniques. For example, the structures 420 can be formedusing an imprint lithography system 100, as described above. Thestructures 420 have a generally symmetric profile. In other words, theshape of one side 422 a of each structure 420 is substantially the sameas the shape of the opposite side 422 b of each structure 420 forming aleft-right symmetry. The starting profile of the structures 420 caninclude any generally symmetric shape including, but not limited to, asquare profile, a rectangular profile, a trapezoidal profile, or atriangular profile. Additionally, the structures 420 can be micro- ornano-structures.

At step (404), a masking material 424 is applied to the structures 420.The masking material 424 is applied under conditions that cause themasking material to asymmetrically cover the structures 420. Forexample, as shown, the masking material 424 completely covers side 422 bof the structures 420, but does not cover side 422 a of the structures420. However, the masking material 424 need not be applied strictly asshown in FIG. 4. The masking material 424 can be applied to cover thestructures 420 in any desired asymmetric pattern. For example, side 422b may be only partially covered by the masking material 424, but notcompletely covered. Side 422 a may be partially covered by the maskingmaterial 424. For example, a portion of both sides 422 a and 422 b maybe covered by the masking material 424. However, to maintain theasymmetric application of the masking material 424 to the structures420, the portion of side 422 a that is covered by the masking material424 may be smaller than the portion of side 422 b that is covered by themasking material 424.

The masking material 424 can be applied using deposition processesincluding, but not limited to, chemical vapor deposition (CVD) andphysical vapor deposition (PVD). Example conditions for asymmetricallyapplying the masking material 424 are described in more detail below inreference to FIGS. 7A-7C. Further, the masking material 424 can includeone of the following materials: Cr, Ti, SiO₂, Al₂O₃, ZrO₂, Ag, Pt, orAu.

At step (406), the structures 420 are etched. Specifically, in theexample shown, the patterned layer 150 is etched to modify the overallshape of the structures 420, thereby, producing an asymmetric profile inthe structures 420. For example, the portions of the structures 420 (andpatterned layer 150) that are not covered by the masking material 424are etched to form recessions 426. The structures 420 can be etchedusing etching processes including, but not limited to, a wet etchprocess, a plasma etching process, a dry etch process, or an ion beametching/milling process.

At step (408), the masking material 424 is removed. For example, themasking material 424 can be stripped from the structures 420 andpatterned layer 150 using a plasma or chemical stripping process. Asshown, removing the masking material reveals the asymmetric profile ofthe structures 420 a produced by the asymmetric application of themasking material 424 and the subsequent etching.

In some implementations, at optional step (410), the asymmetricstructures 420 a are etched a second time to further alter their shape.For example, a second etch without a masking material applied to thestructures can be used to round or smooth the edges of the asymmetricstructures 420 a. The timing and/or techniques used to perform thesecond etch can be altered to produce differing profiles such as profile428 producing asymmetric structures 430 or profile 432 producingasymmetric structures 434. For example, the profile 428 (asymmetricstructure 430) can be produced from asymmetric structures 420 a by usinga relatively lower ion energy etch recipe (e.g., higher pressure, lowerpower) with CHF₃, CF₄, and Ar. The more tapered profile 432 (asymmetricstructure 434) can be produced from asymmetric structures 420 a by usinga relatively higher ion energy etch recipe (e.g., lower pressure, higherpower) with CF₄ and Ar.

FIGS. 7A-7C illustrate an example process for asymmetrically applying amasking material to structures on a substrate in accordance withimplementations of the present disclosure. FIG. 7A shows a simplifiedblock diagram of a deposition system 702. For example, deposition system702 can be a CVD system or a PVD system. The deposition system 702deposits a deposition material 704 (e.g., a masking material) on asubstrate 102. The deposition material 704 is generally in a gaseousphase as it is transferred to the substrate 102. For example, in a CVDsystem the deposition material 704 may be a source gas, while in a PVDsystem the deposition material 704 may be a source material that isevaporated or sputtered onto the substrate 102.

The deposition system 702 directs the deposition material 704 onto thesubstrate along a general deposition direction 706 (e.g., along they-axis as shown). One of skill in the art will appreciate that while thedeposition material 704 does not truly travel along a single straightpath due to the random motions of gas molecules, a general depositiondirection can be determined. That is, the deposition material 704 isgenerally directed towards the substrate from a narrow range ofdirections; not omnidirectionally. For simplicity, however, thedeposition direction 706 will be referred to as a straight path.

The substrate 102 is inclined relative to the deposition direction 706.More specifically, the substrate 102 is inclined such that the substrate102 forms a non-normal angle to the deposition direction 706 of thedeposition system. Inclining the substrate 102 at such an angle causethe deposition material 704 to be applied asymmetrically to anystructures that extend outward from the substrate 102 (e.g., structures420 of FIG. 4). For example, as shown in enlarged region “A” of thesubstrate, when inclined one side of the structures 708 blocks orshadows the opposite side of the structures 708 from the path of thedeposition material 704. The deposition material is thereforepreferentially deposited on the unblocked side of the structures 708.

In some implementations, the substrate 102 can be mounted on a pilotableplatform that allows the substrate 102 to be inclined at a variety ofangles relative the deposition direction 706. In multi-step fabricationprocesses (as described below with respect to FIG. 5) this may permitthe application of a first masking material at a first incline angle andsubsequent masking materials at different incline angles. Furthermore,the substrate 102 may be pivoted about multiple axes to producedifferent structure geometries by asymmetrically masking different sidesof the structures 708. For example, while the substrate 102 shown inFIG. 7A is illustrated as being inclined about the x-axis extending outof the page, for subsequent applications of masking material thesubstrate 102 may be pivoted about the z-axis.

Referring to FIGS. 7B and 7C, in some implementations, the depositionsubstrate may include multiple different regions 730 a, 730 b, 730 c ofstructures. For such implementations, the deposition system 702 caninclude a mask or shutter 732 to preferentially apply masking materialsto one or more particular regions (e.g., region 730 a) while preventingthe deposition material 704 from contacting the other regions (e.g.,region 730 b, 730 c). Either the substrate 102 or mask/shutter 732 canbe rotated remotely to alternate between which regions 730 a, 730 b, 730c are exposed to the deposition material 704 without the need to breakpressure or temperature conditions of the deposition system 702 orwithout the need to alter the incline angle of the substrate 102. Forexample, the substrate 102 can be remotely rotated to alternately aligneach region 730 a, 730 b, 730 c of structures with the window 734 in themask/shutter 730. In some implementations, the mask/shutter 732 maypermit application of deposition material 704 to each region 730 a, 730b, 730 c of structures using a different incline angle for the substrate102 without the need to break pressure or temperature conditions of thedeposition system 702.

In some implementations, process 400 can be repeated as shown in FIG. 5and discussed below. FIG. 5 illustrates a second example process 500 forfabricating asymmetric structures. Process 500 is similar to process400, however, process 500 includes multiple iterations of the some ofthe steps of process 400.

At step (502), a plurality of discrete structures 520 are formed on asubstrate 102. The structures 520 can be formed by, for example, imprintlithography, photo lithography and etching, or other appropriatefabrication techniques. For example, the structures 520 can be formedusing an imprint lithography system 100, as described above. Similar tostructures 420 discussed above, the structures 520 have a generallysymmetric profile. Additionally, the structures 520 can be micro- ornano-structures.

At step (504), a masking material 522 is applied to the structures 520.The masking material 522 is applied under conditions that cause themasking material to asymmetrically cover the structures 520. Forexample, as discussed above with respect to masking material 424, themasking material 522 need not be applied strictly as shown in FIG. 5.The masking material 522 can be applied to cover the structures 520 inany desired asymmetric pattern. The masking material 522 can be appliedusing deposition processes including, but not limited to, chemical vapordeposition (CVD) and physical vapor deposition (PVD). For example, asdiscussed above with respect to FIGS. 7A-7C, the masking material 522can be applied to the structures 520 by inclining the substrate 102 at anon-normal incline angle to the direction from which the maskingmaterial is being deposited. Further, the masking material 522 caninclude one of the following materials: Cr, Ti, SiO₂, Al₂O₃, ZrO₂, Ag,Pt, or Au.

At step (506), the structures 520 are etched. Specifically, in theexample shown, the patterned layer 150 is etched to modify the overallshape of the structures 520, thereby, producing an asymmetric profile inthe structures 520. For example, the portions of the structures 520 (andpatterned layer 150) that are not covered by the masking material 522are etched to form recessions 524. The structures 520 can be etchedusing etching processes including, but not limited to, a wet etchprocess, a plasma etching process, a dry etch process, or an ion beametching/milling process.

At step (508), the masking material 522 is removed and a second maskingmaterial 526 is applied to the, now, modified structures 520 a. As notedabove, the masking material 522 can be stripped from the structures 520and patterned layer 150 using a plasma or chemical stripping process. Asshown, the modified structures 520 a have an asymmetric profile producedby the asymmetric application of the masking material 522 and thesubsequent etching. The shape of the modified structures 520 a can befurther altered by applying a second masking material 526 to themodified structures 520 a. The second masking material 526 can be thesame material or a different material from that of the first maskingmaterial 522.

The second masking material 526 can be applied similarly to the firstmasking material (e.g., as shown). In other words, the second maskingmaterial can be applied asymmetrically to the same side of thestructures 520 a that the first masking material was applied to thestructures 520. In some implementations, the second masking material canbe applied asymmetrically to a different side of the structures 520 a orin different proportions to various sides of the structures 520 a thanhow the first masking material 522 was applied to the structures 520.For example, as discussed above, the first masking material 522 can beapplied to the structures 520 by inclining the substrate 102 at anon-normal incline angle to the direction from which the maskingmaterial is being deposited. Accordingly, the second masking material526 can be applied by inclining the substrate 102 at an angle differentfrom that used to apply the first masking material 522. The substrate102 can be rotated or inclined about a different axis such that thesecond masking material 526 is applied to a different side of thestructures 520 a than that to which the first masking material 522 wasapplied to the structures 520.

At step (510), the structures 520 a are etched. Specifically, in theexample shown, the patterned layer 150 is etched to modify the overallshape of the structures 520 a, thereby, further modifying the asymmetricprofile of the structures 520 a. For example, the portions of thestructures 520 a (and patterned layer 150) that are not covered by themasking material 526 are etched to form recessions 528. The structures520 a can be etched using etching processes including, but not limitedto, a wet etch process, a plasma etching process, a dry etch process, oran ion beam etching/milling process.

At step (512), the masking material 526 is removed. For example, themasking material 526 can be stripped from the structures 520 andpatterned layer 150 using plasma or chemical stripping. As shown,removing the masking material reveals the asymmetric profile of thestructures 520 b produced by the asymmetric application of the maskingmaterial 526 and the subsequent etching.

The optional step (408) of process 400 can be performed either afterstep (512), after the first masking material 522 is removed in step(506), or both. More specifically, either the asymmetric structures 520a, 520 b, or both can be etched without a masking material applied tofurther alter their shape.

FIG. 6 illustrates a third example process 600 for fabricatingasymmetric structures. Process 600 includes the use of an asymmetricallyapplied metal assisted chemical etching (MACE) material for thefabrication of asymmetric structures. As in processes 400 and 500, aplurality of discrete structures 620 are formed on a substrate 102. Thestructures 620 can be formed by, for example, imprint lithography, photolithography and etching, or other appropriate fabrication techniques.Similar to structures 420 and 520 discussed above, the structures 620have a generally symmetric profile. Additionally, the structures 620 canbe micro- or nano-structures.

At step (602), a masking material 622 and a MACE catalyst material 624are applied to the structures 620. The masking material 622 and acatalyst material 624 are applied under conditions that cause maskingmaterial 622 and a catalyst material 624 to asymmetrically cover thestructures 620. For example, the masking material 622 and catalystmaterial 624 can be applied using deposition processes including, butnot limited to, CVD and PVD processes. Example conditions forasymmetrically applying the masking material 622 and catalyst material624 are described in more detail below in reference to FIGS. 7A-7C.Further, the masking material 622 can include one of the followingmaterials: Cr, Ti, SiO₂, Al₂O₃, ZrO₂, Ag, Pt, or Au. The catalystmaterial 624 can include one of the following materials: Au, Pt, Au—Pdalloy,

In some implementations, the masking material 622 and the catalystmaterial 624 can be applied under different conditions to producedifferent asymmetric patterns, as shown in FIG. 6. For example, themasking material 622 may be applied by inclining the substrate 102 at afirst angle relative to a deposition direction of a deposition system,and the catalyst material 624 can be applied by inclining the substrate102 at a second angle that is different from the first angle.

In some implementations, the catalyst material 624 may be applied to thestructures alone, without the masking material 622. Furthermore,although the masking material is described and shown as being applied tothe structures before the catalyst material 624, in someimplementations, the catalyst material 624 may be applied before themasking material 622.

At step (604), the structures 620 are etched. Specifically, in theexample shown, the patterned layer 150 is etched to modify the overallshape of the structures 620, thereby, producing an asymmetric profile inthe structures 620. For example, the portions of the structures 620 (andpatterned layer 150) that are not covered by the masking material 622but which are covered by the catalyst material 624 are etched to formrecessions 626. The catalyst material 624 causes those portions of thestructures 620 and patterned layer 150 that are in contact with thecatalyst to be etched at a higher rate than the structures 620 andpatterned layer 150 alone.

At step (606), the masking material 622 and any remaining catalystmaterial 624 are removed. For example, the masking material 622 andremaining catalyst material 624 can be stripped from the structures 620and patterned layer 150 using a plasma or chemical stripping process. Asshown, removing the masking material 622 and remaining catalyst material624 reveals the asymmetric profile of the structures 620 a produced byprocess 600.

FIG. 8 shows a flowchart of an example method 800 for fabricatingasymmetric structures in accordance with implementations of the presentdisclosure. The process 800 is illustrated as a collection of referencedacts arranged in a logical flow graph. The order in which the acts aredescribed is not intended to be construed as a limitation, and anynumber of the described acts can be combined in other orders and/or inparallel to implement the process.

A plurality of discrete structures are formed on a substrate (802). Thestructures can be formed by, for example, imprint lithography, photolithography and etching, or other appropriate fabrication techniques.For example, the structures can be formed using an imprint lithographysystem 100, as described above. In some examples, the structures have agenerally symmetric profile. In other words, the shape of one side ofeach structure is substantially the same as the shape of the oppositeside of each structure, thereby, forming a left-right symmetry. Thestarting profile of the structures can include any generally symmetricshape including, but not limited to, a square profile, a rectangularprofile, a trapezoidal profile, or a triangular profile. Additionally,the structures can be micro- or nano-structures.

In some implementations, the structures are formed in multiple separateregions. For example, multiple fields of structures can be formed on asingle substrate. In some cases, each region of structures is processedsimilarly to form similarly shaped asymmetric structures. In othercases, each region of structures is processed differently to formdifferently shaped asymmetric structures on the same substrate.

A masking material is asymmetrically applied to the structures (804).The masking material is applied under conditions that cause the maskingmaterial to form an asymmetric masking pattern that asymmetricallycovers the structures. For example, for each or some of the structures,the masking material may completely cover a first side of a structures,but may not cover a second, opposite side of the structure. The maskingmaterial can be applied to cover the structures in any desiredasymmetric pattern. For example, a first side of the structures may beonly partially covered by the masking material, but not completelycovered. A second, opposite side may also be partially covered by themasking material. However, to maintain the asymmetric application of themasking material to the structures, the portion of the first side thatis covered by the masking material may be smaller than the portion ofsecond side that is covered by the masking material.

The masking material can be applied using deposition processesincluding, but not limited to, CVD and PVD processes. Conditions forasymmetrically applying the masking material can include inclining thesubstrate at a non-normal angle to a direction from which depositionmaterial is transferred to the substrate in a deposition system (e.g.,as described above in reference to FIGS. 7A-7C). Further, the maskingmaterial can include one of the following materials: Cr, Ti, SiO₂,Al₂O₃, ZrO₂, Ag, Pt, or Au. In some implementations, a MACE catalystmaterial can be asymmetrically applied to the structures in addition toor in place of the masking material (e.g., as described in reference toprocess 600 above).

Unmasked portions of the structures are etched (806). The structuresand/or layer(s) of material below the structures is etched to modify theoverall shape of the structures, and thereby, produce an asymmetricprofile in the structures. For example, the portions of the structuresthat are not covered by the masking material are etched to formrecessions in the structures and/or a residual layer of material belowthe structures. The structures can be etched using etching processesincluding, but not limited to, a wet etch process, a plasma etchingprocess, a dry etch process, or an ion beam etching/milling process.

The masking material is removed from the structures (808). For example,the masking material can be stripped from the structures and underlyinglayer(s) using a plasma or chemical stripping process. Removing themasking material reveals the asymmetric profile of the structuresproduced by the asymmetric application of the masking material and thesubsequent etching.

In some implementations, the process returns to step (804) and steps(804)-(808) are repeated, as indicated by dashed line 809. For example,as discussed with respect to process 500 above, the steps (804)-(808)can be repeated to further alter the asymmetric shape of the structures.For example, another masking material can be applied under differentconditions to produce a different asymmetric masking pattern.

The structures are, optionally, etched without a masking materialapplied (810). For example, a second etch without a masking materialapplied to the structures can be used to round or smooth the edges ofthe asymmetric structures. The timing and/or techniques used to performthe second etch can be altered to produce differing profiles.

Asymmetric micro- and nano-structures can be used to create diffractionpatterns for use in optical devices. For example, diffraction patternsthat include asymmetric structures may provide more efficient opticaldiffraction patterns for devices such as diffraction lenses or opticalcouplers used in optical wave guides.

FIGS. 9A-9B show example devices in which asymmetric structures areused. FIG. 9A shows a perspective of example an optical system 900. Theoptical system 900 is, for example, an optical projection systemillustrated as a pair of virtual reality or augmented reality glasses.The example optical system can include diffraction lenses and couplersto project an image on a lens 904 of the system 900. The system 900 canreceive data representing image (e.g., from a processor) and project theimage onto a region 902 on a lens 904 of the system 900. Accordingly, auser can view both an image projected in the region 902 as beingoverlaid on a scene that is visible through the lenses 904. Otherexample projection systems can include, but are not limited to, videoprojectors, mobile video projectors, heads-up displays (e.g., heads-updisplays for vehicles), microscopes, telescopes, and other opticaldevices. In other example optical systems 900 asymmetric structures canbe used in reflective polarizer films (e.g., GLAD Wire Grid Polarizers).For example, asymmetric structures can be used in reflective polarizingfilms for LCD display systems such as those used in smartphones, LCDmonitors, LCD televisions, tablet computers, etc.

FIG. 9B shows a top view of a waveguide 950 for projecting an imagewithin a lens 952 that can be positioned in front of a user's eye. Forexample, waveguide 950 can be attached to a pair of glasses 954 toprovide augmented reality images to the user. The waveguide 950 receivesimage data from a processor and projects the an image within the lens952 of the waveguide 950.

Diffraction lenses and optical couplers in the projection system 900 andthe waveguide 950 can include diffraction patterns with asymmetricmicro- and/or nano-structures (as disclosed above) to improve thediffraction efficiency of such lenses and optical couplers. For example,improved diffraction efficiency may result in brighter, more visibleimages to a user. Improved diffraction efficiency may also result inenergy savings for augmented reality and other optical systems.

Although diffraction patterns are described in reference to opticalsystems, it should be understood that implementations of the presentdisclosure are not limited to diffraction patterns for visible light.Instead, the micro- and nano-structures described herein and processesto fabricate the same can be used to produce diffraction patterns forvarious electromagnetic waves having wavelengths corresponding tofeatures of the fabricated structures. For example, the micro- andnano-structures described herein may be functional in diffractionpatterns for electromagnetic waves ranging from infrared (IR)wavelengths to ultraviolet (UV) wavelengths, and potentially to X-rays.

While a number of examples have been described for illustrationpurposes, the foregoing description is not intended to limit the scopeof the invention, which is defined by the scope of the appended claims.There are and will be other examples and modifications within the scopeof the following claims.

1. (canceled)
 2. An optical device comprising: a substrate; and adiffraction pattern on the substrate, the diffraction pattern comprisinga plurality of asymmetric structures extending from a common surface ofthe substrate, wherein each structure of the plurality of asymmetricstructures has a first side surface and a second side surface oppositethe first side surface, and a profile of the first side surface isasymmetric with respect to a profile of the second side surface.
 3. Thedevice of claim 2, wherein each structure of the plurality of asymmetricstructures has an asymmetric triangular profile.
 4. The device of claim2, wherein each structure of the plurality of asymmetric structures hasan asymmetric chair-like profile comprising a linear profile of thefirst side surface and an angled profile of the second side surface. 5.The device of claim 2, wherein each structure of the plurality ofasymmetric structures has an asymmetric stepped profile comprising alinear profile of the first side surface and a stepped profile of thesecond side surface.
 6. The device of claim 2, wherein a recess isdefined between adjacent structures of the plurality of asymmetricstructures.
 7. The device of claim 6, wherein a surface of the recess issubstantially parallel to a surface of the substrate.
 8. The device ofclaim 2, wherein the asymmetric structures comprise one of Si, SiO₂, apolymer material, or an organic-inorganic hybrid material.
 9. The deviceof claim 2, wherein the structures are nano-structures.
 10. The deviceof claim 2, wherein the structures are micro-structures.
 11. The deviceof claim 2, wherein the plurality of asymmetric structures include aplurality of first asymmetric structures having a first asymmetricprofile and a plurality of second asymmetric structures having a secondasymmetric profile that is different from the first asymmetric profile.12. The device of claim 2, wherein each structure of the plurality ofasymmetric structures has rounded edges.
 13. The device of claim 2,wherein the device is a waveguide.
 14. The device of claim 2, whereinthe device is an optical projection system.
 15. The device of claim 14,wherein the device is a virtual reality system or an augmented realitysystem.
 16. The device of claim 14, wherein the device is a pair ofeyeglasses.
 17. A virtual or augmented reality system comprising theoptical device of claim
 2. 18. An optical device comprising: asubstrate; and a diffraction pattern on the substrate, the diffractionpattern comprising a plurality of asymmetric structures extending from acommon surface of the substrate, wherein the plurality of asymmetricstructures are formed by a process comprising: applying a metal catalystlayer over the plurality of discrete structures under first conditionsthat cause the metal catalyst layer to asymmetrically cover theplurality of discrete structures such that at least a first portion of afirst side of each structure of the plurality of discrete is free of themetal catalyst layer; after applying the metal catalyst layer, applyinga masking material over the plurality of discrete structures undersecond conditions that cause the masking material to asymmetricallycover the plurality of discrete structures such that at least a secondportion of a second side of each structure is free of the maskingmaterial, wherein the masking material comprises one of SiO₂, Al₂O₃, orZrO₂; etching a first area of the plurality of discrete structures thatis not covered by the masking material; stripping the masking materialfrom the plurality of discrete structures; and wet etching a second areaof the plurality of structures with metal assisted chemical etching toyield the asymmetric structures.
 19. The device of claim 18, wherein thedevice is an optical projection system.
 20. The device of claim 19,wherein the device is a virtual reality system or an augmented realitysystem.
 21. A virtual or augmented reality system comprising the opticaldevice of claim 17.